Support structure for supporting a moving web-forming wire in a forming zone

ABSTRACT

Disclosed is a support structure for supporting a moving webforming wire in a forming zone. The structure consists of a plurality of very narrow support bars extending across the width of the forming zone for supporting the wire. The bars are spaced apart to form passageways for drainage of slurry water or other fiber suspending fluid from the web. The bars have a large depth dimension compared to the width and, therefore, are capable of withstanding the drainage resistance and weight forces from the wire without excessive deflection, while also creating little obstacle to drainage flow through the support structure. Lateral reinforcement of the support bars is provided by very thin members extending between bars and having a depth dimension almost equal to the depth dimension of the support bars. The narrowest surfaces of the lateral-reinforcing members face the drainage flow through the support so that the members do not significantly interfere with drainage flow. The unique design of the invention provides a very high percentage of open area for drainage flow, greater than 60 percent of the total structure surface area projected on a plane perpendicular to the direction of drainage flow through the structure, and preferably between 70 and 90 percent. It also provides a support structure with no drainage obstructing member large enough to deleteriously affect uniform formation of the web, the widest member being less than 1/2 inch, and preferably 1/8 inch or less.

United States Patent [191 Kaiser et al.

[ SUPPORT STRUCTURE FOR SUPPORTING A MOVING WEB-FORMING WIRE IN A FORMING ZONE [75] Inventors: Richard B. Kaiser, Swarthmore;

Werner A. Schoen, Springfield, both of Pa.

[73] Assignee: Scott Paper Company, Philadelphia,

[22] Filed: June 19, 1972 21 Appl. No.: 264,355

Primary Examiner--S. Leon Bashore Assistant Examiner-Richard V. Fisher Attorney, Agent, or Firm-R. Duke Vickrey; W. J. Foley [111 I 3,821,077 [451, June 28, 1974 57] ABSTRACT Disclosed is a support structure for supporting a moving web-forming wire in a forming zone. The structure consists of a plurality of very narrow support bars extending across the width of the forming zone for supporting the wire. The bars are spaced apart to form passageways for drainage of slurry water or other fiber suspending fluid from the web. The bars have a large depth dimension compared to the width and, therefore, are capable of withstanding the drainage resistance and weight forces from the wire without excessive deflection, while also creating little obstacle to drainage flow through the support structure. Lateral reinforcement of the support bars is provided by very thin members extending between bars and having a depth dimension almost equal to the depth dimension of the support bars. The narrowest surfaces of the lateral-reinforcing members face the drainage flow through the support so that the members do not significantly interfere with drainage flow. The unique design of the invention provides a very high percentage of open area for drainage flow, greater than 60 percent 1 of the total structure surface area projected on a plane perpendicular to the direction of drainage flow through the structure, and preferably between 70 and 90 percent. It also provides asu pport structure with 1 no drainage obstructing member large enough to deleteriously affect uniform formation of the web, the widest member being less than /2 inch, and preferably ls inch or less.

19 Clairm, 11 Drawing Figures PATENTEDwuzs I974 382L077 sum u on;

1 SUPPORT STRUCTURE FOR SUPPORTING A MOVING WEB-FORMING WIRE IN A FORMING ZONE BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to apparatus for forming fibrous webs from aqueous slurries of fibers and water or from other fiber suspending fluids. More particularly, the invention relates to a support structure for supporting a moving foraminous web-forming surface and for facilitating uniform drainage of the slurry water or other fiber suspending fluid through the web-forming surface. The preferred use of the invention is with aqueous slurries, and the description is primarily directed toward that use.

2. Brief Description of the Prior Art Fibrous webs formed from aqueous slurries are typically formed by depositing the slurry upon a moving foraminous web-forming surface and draining the major portion of the slurry water through the forming surface. The forming surface, which is typically a woven wire or plastic fabric, passes over a support structure designed to support the wire and facilitate drainage. The geometric design of the support structure has a significant effect upon drainage from the forming surface, both as to flow rate and to flow uniformity. It is with the gemetric design of the support structure that the invention is concerned.

Support structures typically consist of a structural member or members for supporting the web-forming wire and passageways through the member or between the members for draining slurry water through the support structures. One type of support structure (exemplified in US. Pat. No. 3,425,901 consists of relatively wide support bars extending parallel across the webforming surface perpendicular to the direction of wire travel. These wide support bars can disrupt the uniformity of a web formed over them because they obstruct drainage flow through the support structure. Obstruc tions in the support structure prevent the slurry water from draining straight through the support structure, causing the slurry water to flow sideways around the obstructions. The fibers suspended in the water flowing around the obstruction are moved sideways and are deposited nonuniformly over the web surface.

It is known that reducing the size of the support bars in the direction of wire travel could reduce the deleterious forming effects on the web. But, the sizes of the bars in prior art support structures have generally been made relatively wide to prevent the excessive vertical deflection which would otherwise occur in the long bars, which may span up to 200 inches and more across the web-forming zone. The widths of the prior art sup port bars are typically more than /2 inch, and generally several inches. Increasing the depth of the bars would enable support bars narrower than this to resist excessive vertical deflection, but .it has been believed that lateral stability requires wide bars.

In some cases, the parallel support bars are supported by elaborate under-structure designed to eliminate excessive vertical deflection. Examples of such understructure are disclosed in US. Pat. Nos. 3,357,881 and 3,585,105. Other types of parallel-bar support structures also include lateral-reinforcing members extending under or between the support bars. Examples are 2 disclosed in US. Pat. Nos. 3,265,560; 3,266,977 and 3,337,394. These under-structures and lateralreinforcing members in the prior art support structures present substantial obstructions to drainage, and there fore, do not allow satisfactory drainage flow uniformity.

Another type of support structure is disclosed in US. Pat. Nos. 2,976,925; 3,136,685 and 3,365,359, This type of structure is a solid plate having uniformly distributed drainage openings through it. US. Pat. No. 3,262,842 discloses a similar type of support structure, but one which consists of an assembly of blocks having spacings between the blocks to form the drainage passageways.

All of these prior art types of support structures can be employed satisfactorily toward the end of the webforming process where the fibers of the web have already taken a fixed position with respect to each other. At this stage, the amount of water to be removed is generally small, and a large amount of open area is not required. Furthermore, the support structures are gener ally used at this stage in conjunction with suction boxes developing substantial vacuum beneath the structure, thus enabling a small amount of open area to be adequate for drainage.

However, at the initial web-forming stage where drainage uniformity is very important, the support structures of the prior art are not completely satisfactory. This is especially true when the slurry contains long fibers, such as rayon, other man-made fibers, or natural fibers. Such long fibers are difficult to deposit uniformly on a forming wire. Furthermore, they are generally in a very dilute slurry, requiring drainage of a large quantity of water. The amount of open area in the prior art types of support structure is not sufficient to quickly drain a large quantity of water. They generally have no more open area than about 50 percent of the total surface area of the support structure, and typically much less than that. Furthermore, their support members create large drainage obstruction areas and widely separated vacuum drainage zones, both of which can prevent uniform formation of the web.

The support structures of the prior art canbe modi fied to increase the amount of open area by increasing the spacing between the bars or the size of the perforations. But excessive spacing allows the forming wires to sag between the supports. Sagging greatly increases wear on the forming wire and increases power requirements to move the wire. Sagging may also undesirably alter the distribution pattern of the fibers in the web.

Having described the shortcomings of the prior art,

it is an object of the inventionto provide a support structure for a web-forming wire, which has a very large percentage open area for drainage. It is also an object of the invention to provide, a support structure for a web-forming wire, which has spacing between support elements small enough to avoid excessive sagging of the forming wire. A further object of the invention is to provide a support structure for a web-forming wire, which has support elements that are narrow enough to avoid deleterious forming effects on a rapidly formed web.

BRIEF SUMMARY OF THE INVENTION wide) support bars extending across the width of the forming zone for supporting the wire. The bars are spaced apart to form passageways for draining the slurry water or other fiber-suspending fluid. The spacing between the bars is small enough to prevent excessive sagging of the forming wire between supports. The width of support bars in the direction of wire travel is small enough to eliminate excessive disruptive forming effects on the web caused by the support bars blocking drainage. The open area between the support bars available for drainage is a very large percentage of the total structure surface area projected on a plane perpendicular to the direction of drainage flow through the structure, greater than 60 percent and preferably between 70 and 90 percent. The large percentage of open area is possible even with the short spacings between the support bars, because the width of the bars is small.

Excessive deflection of the support bars is prevented by the large depth (compared to the width) of the support bars. Typically the bars are several inches deep. Lateral reinforcement of the support bars is provided by very thin members extending from each support bar to its adjacent bars to enable all of the bars to act as a unit in resisting lateral forces, such as those caused by the forming wire moving across the top of the structure. The lateral-reinforcing members, are arranged where their narrowest surfaces face the drainage flow through the passageways so that they do not significantly decrease the amount of open area available for drainage flow. Yet, because of their large dimension in the drainage flow direction, they offer adequate lateral reinforcement to the support bars.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates, in a sectional elevation view, a preferred form of the invention supporting a forming wire in a web-forming zone.

FIG. 2 is a fragmentary plan view of the support structure of the invention taken along line 22 of FIG. 1.

FIGS. 3 through 6 illustrate alternative forms of lateral-reinforcing members for the support structure of the invention.

FIGS. 7 through 9 illustrate alternative forms of the invention.

FIG. 10 is a sectional elevation view of the support structure of the invention supporting a forming wire in a web-forming zone and illustrates a particular feature of the invention.

FIG. 11 is a sectional elevation view of an alternative embodiment to a support structure of the invention supporting a forming wire in a web-forming zone.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A typical web-forming operation from an aqueous slurry is illustrated in FIG. I, where a slurry 15 of water and fibers is conducted by a slice 16 to a web-forming surface provided by woven wire fabric 17. The majority of the water in the slurry 15 passes through the forming wire 17, leaving the fibers deposited upon the forming wire 17 to form a web 18. The forming wire 17 continuously moves through the web-forming zone, carrying the newly formed web 18 to other parts of the papermaking machine.

The forming wire 17, while passing through the webforming zone, is supported by the support structure 19 of the invention mounted upon ledges 14 at each side of the web-forming zone. The support structure 19 not only must support the forming wire 17 in moving contact, but must also facilitate water drainage from the slurry 15 through the forming wire 17. To satisfy these requirements, the support structure 19 consists of spaced apart, parallel support bars 20 extending across the full width of the forming wire 17 perpendicular to the direction of web travel. The bars 20 have a very small width (indicated as w" in FIGS. 1 and 2) extending in the direction of wire 17 movement. This width w is less than /2 inch, and preferably Vs inch or less.

Between the bars 20 are passageways 21 for flow of the drainage water. The necessary drainage flow area available through the passageways 21 to enable rapid formation of the web 18 is provided by spacing the bars 20 far enough apart to give a large percentage of open area compared to the total structure surface area projected on a plane perpendicular to the direction of drainage flow through the structure, more than percent and preferably between percent to percent. At the same time, the bars 20 are spaced so that the spacing S between adjacent bars (the maximum unsupported span of the forming wire 17 in the direction of wire movement) is not so great that it causes excessive sagging of the forming wire 17 between the bars 20. The maximum spacing S which can be tolerated without excessive sagging varies with the wire configuration and operating factors, but generally it is less than 2 inches.

The bars 20 are often required to be quite long to extend across the width of a large papermaking machine. To prevent excessive deflection in the middle of the forming surface, the depth D of the bars 20 is relatively large compared to the width w, at least four times as large and often 16 times or more, depending upon the length L of the bars 20 extending across the width of the web forming zone. This unique bar design offers a support bar which both resists deflection in the direction perpendicular to the forming wire and also causes very little impediment to drainage flow.

The support bars 20 of the invention are capable of resisting deflecting forces in-the direction perpendicular to the forming wire 17. But, without additional reinforcement they are not capable of resisting forces in the lateral direction (direction of wire movement), such as the frictional drag forces caused by the moving wire 17. To impart deflection resistance in the lateral direction, the invention employs lateral-reinforcing members 22. The members 22 extend from each bar 20 to its adjacent bars 20 and enable all of the bars 20 to resist lateral forces as a unit, rather than individually.

The members 22 are provided by very thin sheet material (from about 1/64 inch to /8 inch thick) which has its narrowest surface facing the direction of drainage flow, so that they present very little impediment to drainage flow through the passageways 21. At the same time, the members 22 extend in the drainage flow direction for approximately the full depth of the support bars 20, a design which gives maximum lateral reinforcement with minimum drainage flow restriction. It is preferable to extend the support bars 20 beyond the lateral-reinforcing members 22 to support the wire 17, thus avoiding contact between the moving wire 17 and The lateral-reinforcing members illustrated in FIG. 2 are indicated generally by numeral 22a and are shown as obliquely spanning the open space between support bars 20. The reinforcing members 22a preferably include segments 23a to facilitate their attachment to the support bars 20 by welding, bolting or other suitable method.

FIGS. 3 through 6 illustrate alternative embodiments for the lateral-reinforcing members 22. In FIG. 3, the members 22b extend perpendicularly between support bars 20 and have attachment segments 23b. In FIG. 4, the members 220 are circular and are attached to the support bars 20 and to each other at the tangential contact points. In FIG. 5, the members 22d are octagonal honeycomb-type structures attached to the support bars and to each other at adjoining flat sides. FIG. 6 illustrates lateral-reinforcing members 222 which are similar to the members in FIG. 5, but are hexagonal honeycomb-type structures.

In all of the illustrated embodiments, the lateralreinforcing members 22 have a wall thickness which is very thin, preferably between 1/64 inch and 42 inch. The arrangement and size of the lateral-reinforcing members is such that they obstruct only a small portion (less than 20 percent) of the drainage flow area 21 between the support bars 20. In this manner, they provide adequate lateral reinforcing without significantly blocking drainage flow area. Also, they may be arranged in a staggered pattern in the web movement direction to further minimize any undesirable webforming effect they might have.

The support bars 20 are typically straight bars, very thin and relatively deep compared to their width. However, they may be varied somewhat and still be within the scope of the invention. For example, in FIG. 7 the bars 20 are illustrated as all having portions above the upper edge of the lateral-reinforcing members which are bent in the same direction. This arrangement may be beneficial where the water from the slurry l5 flows through the forming wire 17 at an acute angle, because it delays changing the flow direction of the drainage water until it is beyond the forming wire 17, thus avoiding possible web-forming disturbances which might not be'desired.

Referring to FIGS. 1 and 7, the wire-support surface 24 of the support bars 20 is provided by the upper edge of the support bars 20. Preferably, the wire-support surface 24 is treated to resist wear by one of many methods known to those skilled in the art, one example of which is described in U.S. Pat. No. 3,351,524.

The support bars 20 of the invention are readily adaptable to mounting removable wire-support members, if desired, to the upper edge of thesupport bars 20. Such wire-support members are illustrated in FIG. 8 by numeral fand in FIG. 9 by numeral 25g and are to be considered as part of the support bars 20 for purposes of defining and claiming the invention. Typically, the removable wire-support members are continuous strips of low friction material (nylon, for example) which extend across the full length L" of the support bars 20. The wire-support members can be slotted for conveniently slipping over the support bars 20 or they can be attached by other methods. In all casesthe width W of the wire-support members is small, and as it affects drainage uniformity and the amount of open area available for drainage flow, it is always less than /2 inch wide, and leaves an amount of open area in the support structure greater than 60 percent.

FIG. 9 illustrates another embodiment of the invention. In this embodiment, the wire-support members 25g are shaped as foils to enhance drainage. Their shape includes a trailing surface 29 which extends downwardly from the wire at a few degrees, typically from I to 5. Other shapes could also be used.

FIG. 10 illustrates another feature of the invention. This figure illustrates a slurry 15 of water and fibers conducted by a slice 16 to an inclined web-forming wire 17. The webforming wire 17 is supported in the web-forming zone by the support structure 19 of the invention. In this embodiment of the invention, the support bars 20 extend beneath the lateral-reinforcing members 22. The support 19 is mounted over a series of vacuum boxes 26 (only two are illustrated, but more could be provided if desired). Divider walls 27 of the vacuum boxes 26 extend for the length of the support bars 20 in engagement with selected support bars 20. On top of each divider wall 27 is a resilient pad 28 (of rubber or other suitable material) capable of forming a vacuum seal across the entire length L of the support bar 20 to maintain separation between adjacent vacuum boxes 26, thus enabling varying vacuum levels to be maintained in each box 26.

In all of the embodiments of theinvention the support bars 20 and the lateral-reinforcing members 22 are both to be made from a strong rigid material. A noncorrosive type of metal, such as stainless steel, is particularly desirable where the fiber-suspending fluid is water or other corrosive fluid, but other metals and structural materials could also be used, and may be preferred where non-corrosive fiber suspending fluids such as air are involved.

In the illustrated embodiments of the invention, the support bars 20 have been described as being parallel bars disposed perpendicular to the direction of forming-wire movement. FIGS. 1 through 10 further illustrate them as forming flat planes disposed perpendicular to the plane of the forming wire. It is to be recognized that certain modifications to this description can be made within the scope of the invention.

For one, the bars 20 do not necessarily have to be parallel to each other. But, since the limitation on the maximum span between support bars must be observed, the largest percentage open area for drainage can be obtained by disposing the bars parallel to each other. For another, the length L" dimension of the bars 20 does not have to be disposed perpendicular to the direction of wire movement, although this disposition enables use of the shortest bars and therefore lesser bar depths to prevent vertical deflection. It is to be noted, however, that the critical width dimension w" (or W," whichever is larger) for the bars 20 is to be measured in the direction of wire 18 movement, whether or not the bars are disposed perpendicular to the direction of wire 18 movement. Therefore, a bar 20 of any given thickness will have its smallest w or W dimension when it is disposed perpendicular to the direction of wire 18 movement.

The bars 20 do not necessarily have to be flat planes disposed perpendicular to the plane of the forming wire 17. Other shapes and dispositions could be used. For example, FIG. 11 illustrates an alternative embodiment of the invention where the support bars 20 are not disposed perpendicular to the plane of the forming wire 17. In this figure, the support structure of the invention is shown in the same environment illustrated in FIG. 1. However, in contrast to FIG. 1, the support structure 19 of FIG. 11 includes support bars 20 which are sloped toward the flow direction of the slurry through the slice 16. In this disposition, they form an angle with a plane normal to the plane of the forming wire 17. The angle can be chosen to effect desired forming characteristics of the fiber web 18.

Whatever the shapes or dispositions of the support bars in the support structure 19, the critical dimensions of the structure 19 must be within the requirements of the invention and are defined as follows:

The width of each support bar 20 (which includes the wire support member 25, if one is attached) is the largest dimension of the bar 20 measured in the direction of wire 18 movement. If the width of each wiresupport member (indicated by W in FIGS. 8, 9 and 10) is larger than the width" of each support bar 20 (indicated by w in FIGS. 1, 2 and 11), then the width dimension refers to W. If smaller or there is no wire-support member 25, then the width" dimension refers to w.

The depth of each support bar 20 determines the deflection resistance of the bar to forces exerted on it from the wire in a direction normal to the wire. Therefore, the dimension of each bar 20 is measured in a direction perpendicular to the plane of the forming wire 17, regardless of the disposition of the bars. Thus, dimension D as indicated in FIG. 11 is not measured parallel to the slope of the support bars 20.

The unsupported span" of the web-forming wire 17 (indicated by S in FIGS. 1, 2 and 7 through 11) is measured in the direction of wire travel and represents the distance from the wire supporting end of one support bar 20 to the wire-supporting beginning ofthe next support bar 20.

The minimum unobstructed area in the passageways is measured in a plane perpendicular to the direction of drainage flow through the support structure 19 and is measured at the location where the passageways are smallest. In caseswhere there are wire-supporting members 25 wider than the support bars 20 the minimum unobstructed area might be at the wiresupporting members 25. The minimum unobstructed area" represents the degree to which drainage flow will be unobstructed and is expressed as a percentage of the flow area which would be available absent the physical obstructions of the support structure 19, which is equivalent to the total structure 19 surface area projected on a plane perpendicular to the direction of drainage flow through the structure 19. Where the support bars 20 are disposed perpendicular to the forming wire 17, the projected structure 19 surface area is equivalent to the area of the wire 17 supported by the structure 19. Where the support bars 20 are disposed non-perpendicular to the forming wire 17, the pro jected" structure 19 surface area will be less than the area of the wire 17 supported by the structure 19.

The invention is particularly useful at the initial webforming stage of fibers from an aqueous slurry. It is even more useful where the slurry is very dilute and contains long fibers such as rayon. other man-made fi bers or natural fibers. But it is to be recognized that the invention can also be used satisfactorily toward the'end of the web-forming process in an aqueous slurry process. It is also to be recognized that the invention can be used satisfactorily in a web-forming operation where the fibers are suspended in fluids other than water, such as air for example.

Having described the invention, we claim:

1. A drainage structure for supporting a moving webforming wire in a web-forming zone, comprising:

a plurality of spaced-apart wire-supporting bars extending across the width of the web-forming zone and defining drainage passageways between the bars, and

lateral-reinforcing members extending from each bar to adjacent bars within the passageways and spanning the open space between the adjacent bars;

each bar having a first dimension in the direction of wire movement of less than /2 inch and a second dimension in the direction perpendicular to the plane of the wire which is at least four times as large as the first dimension,

the lateral-reinforcing members comprising thin sheet-like materialnot more than 41 inch thick and .having their narrowest surface facing the drainage flow through the passageways;

the minimum unobstructed area in the passageways being equal to more than 60 percent of the structure surface area projected on a plane perpendicular to the direction of drainage flow through the structure.

2. A drainage structure as recited in claim 1, wherein the bars are spaced from each other to provide a maximum unsupported span of two inches for the webforming wire in the direction of wire movement.

3. A drainage structure as recited in claim 1 wherein the bars are disposed parallel to each other and across the width of the forming zone perpendicular to the direction of wire movement.

4. A drainage structure as recited in claim 2 wherein the minimum unobstructed area in the passageways is equal to between and percent of the structure surface area projected on a plane perpendicular to the direction of drainage flow through the structure.

5. A drainage structure as recited in claim 4 wherein the bars are disposed parallel to each other and across the width of the forming zone perpendicular to the direction of wire movement.

6. A drainage structure as recited in claim 5, wherein the first dimension of each support bar is Vs inch or less.

7. A drainage structure as recited in claim 1 wherein the second dimension is at least l6 times as large as the first dimension.

8. A drainage structure as recited in claim 1 wherein the wire-supporting parts of the bars extend beyond the lateral reinforcing members to support the webforming wire away from the lateral-reinforcing members.

9. A drainage structure as recited in claim 8 including wire-supporting members of low friction material attached to the support bars to support the web-forming wire.

10. A drainage structure as recited in claim 9, wherein the wire-supporting members of low friction material have elongated slots into which portions of the bars are insertable to provide a removable means of attachment to the bars.

11. A drainage structure as recited in claim 8, wherein the wire-supporting parts of the bars have been treated to be more wear resistant than the remaining portions of the bars.

12. A drainage structure as recited in claim 6 wherein a portion of each bar extending beyond the lateral reinforcing members is disposed at an obtuse angle to the remaining portion of each bar.

13. A drainage structure as recited in claim 9, wherein at least some of the wire-supporting members of low friction material have a planar surface extending at an acute angle to the wire.

14. A drainage structure as recited in claim 1, wherein the parts of the bars farthest from the webforming wire extend beyond the lateral reinforcing members whereby a vacuum-tight seal may be provided when the bars are supported by a resilient pad of impermeable material.

15. A drainage structure as recited in claim 3 wherein the lateral reinforcing members extend from bar to bar perpendicular to the plane of the bars.

16. A drainage structure as recited in claim 3, wherein the lateral reinforcing members extend from bar to bar at an oblique angle to the plane of the bars.

17. A drainage structure as recited in claim 3, wherein the lateral reinforcing members are provided by a honeycomb-like structure.

18. A drainage structure as recited in claim 3, wherein lateral reinforcing members are provided by circular structures.

19. A drainage structure as recited in claim 1 wherein the support bars extend from the plane of the forming wire at a non-perpendicular angle 

1. A drainage structure for supporting a moving web-forming wire in a web-forming zone, comprising: a plurality of spaced-apart wire-supporting bars extending across the width of the web-forming zone and defining drainage passageways between the bars, and lateral-reinforcing members extending from each bar to adjacent bars within the passageways and spanning the open space between the adjacent bars; each bar having a first dimension in the direction of wire movement of less than 1/2 inch and a second dimension in the direction perpendicular to the plane of the wire which is at least four times as large as the first dimension, the lateral-reinforcing members comprising thin sheet-like material not more than 1/8 inch thick and having their narrowest surface facing the drainage flow through the passageways; the minimum unobstructed area in the passageways being equal to more than 60 percent of the structure surface area projected on a plane perpendicular to the direction of drainage flow through the structure.
 2. A drainage structure as recited in claim 1, wherein the bars are spaced from each other to provide a maximum unsupported span of tWo inches for the web-forming wire in the direction of wire movement.
 3. A drainage structure as recited in claim 1 wherein the bars are disposed parallel to each other and across the width of the forming zone perpendicular to the direction of wire movement.
 4. A drainage structure as recited in claim 2 wherein the minimum unobstructed area in the passageways is equal to between 70 and 90 percent of the structure surface area projected on a plane perpendicular to the direction of drainage flow through the structure.
 5. A drainage structure as recited in claim 4 wherein the bars are disposed parallel to each other and across the width of the forming zone perpendicular to the direction of wire movement.
 6. A drainage structure as recited in claim 5, wherein the first dimension of each support bar is 1/8 inch or less.
 7. A drainage structure as recited in claim 1 wherein the second dimension is at least 16 times as large as the first dimension.
 8. A drainage structure as recited in claim 1 wherein the wire-supporting parts of the bars extend beyond the lateral reinforcing members to support the web-forming wire away from the lateral-reinforcing members.
 9. A drainage structure as recited in claim 8 including wire-supporting members of low friction material attached to the support bars to support the web-forming wire.
 10. A drainage structure as recited in claim 9, wherein the wire-supporting members of low friction material have elongated slots into which portions of the bars are insertable to provide a removable means of attachment to the bars.
 11. A drainage structure as recited in claim 8, wherein the wire-supporting parts of the bars have been treated to be more wear resistant than the remaining portions of the bars.
 12. A drainage structure as recited in claim 6 wherein a portion of each bar extending beyond the lateral reinforcing members is disposed at an obtuse angle to the remaining portion of each bar.
 13. A drainage structure as recited in claim 9, wherein at least some of the wire-supporting members of low friction material have a planar surface extending at an acute angle to the wire.
 14. A drainage structure as recited in claim 1, wherein the parts of the bars farthest from the web-forming wire extend beyond the lateral reinforcing members whereby a vacuum-tight seal may be provided when the bars are supported by a resilient pad of impermeable material.
 15. A drainage structure as recited in claim 3 wherein the lateral reinforcing members extend from bar to bar perpendicular to the plane of the bars.
 16. A drainage structure as recited in claim 3, wherein the lateral reinforcing members extend from bar to bar at an oblique angle to the plane of the bars.
 17. A drainage structure as recited in claim 3, wherein the lateral reinforcing members are provided by a honeycomb-like structure.
 18. A drainage structure as recited in claim 3, wherein lateral reinforcing members are provided by circular structures.
 19. A drainage structure as recited in claim 1 wherein the support bars extend from the plane of the forming wire at a non-perpendicular angle. 